Microsoft has launched an ad-free, no cost version of its Bing search engine that can be used in public and private schools across the U.S.

Activating ad-free search also activates an automatic filter that blocks adult content and enhances privacy protections, said Microsoft in a statement Wednesday. The service is meant for primary and secondary (K-12) schools with students aged from 5 to 18.

Several schools with more than 800,000 students have already signed up for the pilot program including the Los Angeles Unified School District, Atlanta Public Schools, the Fresno, California, Unified School District and Detroit Country Day School, Microsoft said. Other interested schools can sign up for the pilot, although space is limited, it said.

Microsoft took the opportunity to lash out at its biggest competitor. "When students use Google for searches in school, they are shown ads that can distract from their studies," it said on the Bing for Schools site that is part of Microsoft's "Scroogled" campaign which targets Google's search practices.

A Google spokesman declined to comment on Microsoft's project.

As part of the pilot, people can also earn credits for their favorite schools through the Bing Rewards program by searching the Web from a desktop or mobile device using Bing, Microsoft said. When 30,000 credits are saved the school will receive a Microsoft Surface RT tablet. It will award about 60 tablets monthly.

Bing for Schools also offers three learning activities every school day targeted at children in different age groups between kindergarten and 12th grade, Microsoft said. The activities use the Bing homepage image of the day to pose a question that can be solved using search tools.